How about grocery delivery? Maybe you could get other things delivered at the same time (snacks, diapers, etc.) so you don't have t pay an extraordinary delivery charge for only one thing.
I don't know about POP, but some resorts sell milk by the half gallon... I couldn't find it at the Poly but was able to pick some up at AKL. And all the resorts with vacation villas (BWV, BCV, VWL, OKW, SS0 have groceries with milk.
I'd never buy a whole gallon at once at WDW - where are you going to keep it cold? It won't fit in one of those little refrigerators you can get. Pop Century had small cartons of milk in the food court area.
Thanks for the responses. Wasn't really sure if it is a gal I want I am use to dealing with litres. Anyway I will be at the AKL the second night of my trip for dinner so I can get a 1/2 gal there. Last trip we stayed at the Boardwalk and had no trouble getting it but wasn't sure about the value resorts.